In an LTE (long term evolution) TDD (time division duplex) or FDD (frequency division duplex) system, or a system where TDD and FDD carriers are aggregated, a base station may configure at least two carriers for a UE (user equipment), and transmit data to the UE through at least two carriers, where the configuration is referred to as carrier aggregation. The base station transmits PDCCH (physical downlink control channel) information to the UE in a downlink subframe, where the PDCCH information indicates scheduling information of a PDSCH (physical downlink shared channel) carried in the downlink subframe. The UE determines an uplink subframe according to a timing relationship between the subframe carrying the PDSCH and a subframe carrying an ACK (acknowledgement) or NACK (negative acknowledgement) corresponding to the PDSCH, and feeds back the ACK/NACK corresponding to the PDSCH in the uplink subframe of a primary carrier.
For carrier aggregation, after the base station transmits downlink data to the UE, the UE needs to feed back a corresponding ACK/NACK to the base station, where, if the UE feeds back an ACK, it indicates that data transmission is correct, or if the UE feeds back a NACK, the base station initiates corresponding retransmission.
However, the conventional ACK/NACK transmission method is applicable to only carrier aggregation of at least two TDD carriers with the same uplink and downlink subframe configurations, or carrier aggregation of at least two FDD carriers. When at least two TDD carriers of different uplink and downlink subframe configurations are aggregated, or at least one FDD carrier and at least one TDD carrier are aggregated, especially when a TDD carrier is used as a primary carrier, it is possible that an ACK/NACK cannot be fed back on the primary carrier, and no solution for feeding back the ACK/NACK is available at present.